1. Field of the Invention
The present invention relates to an optical mixer used for an optical communication system, and more particularly, to a technique for compensating for loss that occurs when waveguides intersect each other.
2. Description of the Related Art
With the increase in transmission rates of optical communication systems, communication systems capable of realizing higher capacity, more efficient and faster communications are actively under review (e.g., JP2008-193555A). Among such communication systems, DP-QPSK (Dual-polarization Quadra phase shift keying) has been adopted as the favorite for 100GE transmitting apparatuses.
FIG. 1 is a diagram illustrating an example of a 90° hybrid interferometer, which is a general optical mixer used for an optical communication system.
As shown in FIG. 1, the 90° hybrid interferometer in this example has a feature in which an optical path length of a waveguide outputted from an upper optical coupler of two optical couplers 120 and connected to optical coupler 130b is shifted by π/2 from the other waveguide of the interferometer to which a TE signal is inputted. Furthermore, the 90° hybrid interferometer has another feature in which lengths of waveguides outputted from optical coupler 110a and connected to optical couplers 130a and 130b are equal. The same applies to the interferometer that processes a TM signal, and an optical path length of a waveguide outputted from optical coupler 120 and connected to optical coupler 130c is configured to be shifted by π/2 from the other waveguide of the interferometer to which the TM signal is inputted. Here, the waveguide outputted from optical coupler 120 and inputted to optical coupler 130a is configured to intersect the waveguide outputted from optical coupler 110a and connected to optical coupler 130b at waveguide intersection 140. The same applies to the interferometer that processes the TM signal.
When the 90° hybrid interferometer configured as shown above is used, upon receiving a DP-QPSK signal, 100GE converts output of the 90° hybrid interferometer and eight PD outputs from the 90° hybrid interferometer to digital signals through AD conversion. The signal light is divided into two portions; TE signal and TM signal, which are inputted to the 90° hybrid interferometer independently of each other and which are made to interfere with local light. There is such a feature in which it is possible to compensate for signal degradation generated by wavelength dispersion or polarized mode dispersion by carrying out DSP processing on the digital signals without using any dispersion compensation fiber.
With regard to the 100GE scheme, discussions on various types of MSA are currently underway. One such example is MSA implementation of a receiving module and studies are underway for introducing a polarization beam splitter, 90° hybrid interferometer, eight PDs and TIA into a small casing of 75 mm×35 mm.
The aforementioned 90° hybrid interferometer requires two types of waveguide; a waveguide connected from optical coupler 110a to optical coupler 130a and a waveguide connected from optical coupler 110a to optical coupler 130b whose arm lengths are equalized, and a waveguide connected from optical coupler 120 to optical coupler 130b having a difference in arm length of π/2. Furthermore, with regard to PD outputs, there are waveguide intersections 140 where the waveguide of the TE signal which is branched and outputted by optical coupler 110a intersects the waveguide of the local light outputted from optical coupler 120, and the waveguide of the TM signal which is branched and outputted by optical coupler 110b intersects the waveguide of the local light outputted from optical coupler 120. When there are such waveguide intersections 140 where waveguides intersect each other, intersection loss occurs in one of the waveguides making up the interferometer, which may cause an extinction ratio to degrade. Loss that occurs in an intersecting waveguide is normally on the order of 0.1 to 0.2 dB. When light decreases by 0.2 dB on one arm, the extinction ratio of the interferometer degrades down to the order of a maximum of 13.5 dB. To avoid such degradation and maintain a high extinction ratio of the interferometer, a setting needs to be made such that loss values on both arms are equal.
Here, a technique is conceived whereby one of two waveguides of different optical path lengths is provided with optical path length/loss adjusting means for compensating for loss produced by the difference in optical path length (e.g., JP2002-122895A).
However, the technique whereby one of two waveguides of different optical path lengths is provided with optical path length/loss adjusting means to thereby compensate for loss produced by the difference in optical path length can compensate for loss caused by the difference in optical path length, but there is a problem in which the above described loss caused by the waveguides intersecting each other cannot be compensated.